The White knight's threat of check on d6 is a bit awkward: ...Ke7 would be unpleasant so Black would have to surrender his bishop with ...Bxd6, only to have a queen arrive on d6 and single-handedly cramp the whole Black position. She would need to go, too: ...Qe7 Qxe7 Kxe7/Nxe7, and suddenly Black's dodgy d-pawn and slow development are significant factors in the much simplified position. Far better to keep the knight out of d6 for now and then vanquish it to a3 in a move's time with 6...d6.